Polyimide films have good mechanical properties and other good characteristics such as heat resistance, chemical resistance, and electrical insulation properties and are therefore widely used as various films for optical waveguides and electronic devices such as interlayer dielectric films for semiconductors, buffer coatings, flexible printed circuit substrates, and liquid-crystal alignment films.
At present, glass is widely used for substrates for, e.g., liquid crystal display devices, organic EL display devices, and organic TFTs. However, with the trend toward weight reduction and flexibilization, flexible substrates produced using plastics such as PEN (polyethylene naphthalate) and PES (polyethersulfone) are being developed. The basic characteristics of flexible substrates that can be used as alternatives to glass substrates may include, e.g., high transparency, low thermal expansion properties, high thermal resistance, and low birefringent properties.
The substrate having the basic characteristics must satisfy the following specific requirements. The transmittance of 400 nm light at a film thickness of 10 μm must be 80% or more. In order to prevent improper arrangement of display pixels and wiring traces on a substrate due to expansion and contraction of the substrate, the thermal expansion coefficient must be 20 ppm/° C. or less in the range of 100° C. to 200° C. As to the birefringent properties, the difference between the refractive index in TE (Transverse Electric) mode and the refractive index in TM (Transverse Magnetic) mode must be 0.05 or less. The glass transition temperature must be 250° C. or more.
At present, the most well-known example of practical low-thermal expansion polyimide materials is a polyimide produced from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and p-phenylenediamine. It is known that a film obtained from this polyimide exhibits a very low thermal expansion coefficient (5 to 10 ppm/° C.), although the value may vary depending on the thickness and production conditions (for example, Non-Patent Document 1). However, the light transmittance of this polyimide film is substantially 0% at 400 nm.
It is known that the introduction of a highly flexible monomer or a fluorine substituent into the skeleton is effective to obtain a highly transparent polyimide film. For example, Non-Patent Document 2 reports a perfluoro polyimide obtained from 2,2-bis(3,4-carboxyphenyl)hexafluoropropanoic dianhydride (a fluorinated acid dianhydride) and 2,2′-bis(trifluoromethyl)benzidine (a fluorinated diamine). A film obtained from the aforementioned perfluoro polyimide exhibits a light transmittance as high as 85% when the light is at 400 nm and the thickness of the film is 20 μm. However, the thermal expansion coefficient of the film is as high as 48 ppm/° C., and the film does not satisfy the requirement for the low thermal expansion properties.
Non-Patent Document 2 also reports a polyimide obtained from pyromellitic dianhydride and 2,2′-bis(trifluoromethyl)benzidine. A film obtained from this polyimide exhibits good low thermal expansion properties with a thermal expansion coefficient of 3 ppm/° C. However, the transmittance of 400 nm light through the film having a thickness of 20 μm is 10%, which does not satisfy the requirement for the high light transmittance.
Patent Document 1 reports a polyimide produced from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and trans-1,4′-cyclohexyldiamine. A film produced from this polyimide has good light transmittance such that the 1% cutoff wavelength is 368 nm when the thickness of the film is 5 μm. However, its thermal expansion coefficient is 23 ppm/° C., which does not satisfy the requirement for the low thermal expansion properties.
Patent Document 2 reports a polyimide produced from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and 2,2′-bis(trifluoromethyl)benzidine. In Patent Document 2, a polyimide film obtained using an ordinary thermal conversion method has good light transmitting properties such that the transmittance of 380 nm wavelength light through the film having a thickness of 3 mil (=75 μm) is 78%. However, its thermal expansion coefficient is as high as 38 ppm/° C., which does not satisfy the requirement for the low thermal expansion properties. A polyimide film obtained using a chemical conversion method has good light transmitting properties such that the transmittance of 380 nm wavelength light through the film having a thickness of 3 mil (=75 μm) is 76%, and the film also has low thermal expansion properties such that the thermal expansion coefficient is −3 ppm/° C. However, the chemical conversion method essentially requires a step of removing a catalyst upon forming the film, and therefore this method is not suitable for mass production.
Patent Document 3 reports a polyimide produced from 3,3′,4,4′-dicyclohexyl tetracarboxylic dianhydride and 4,4′-diaminodiphenyl ether. In Patent Document 3, a polyimide film obtained using an ordinary thermal conversion method cured at 300° C. and having a thickness of 35 μm has a good light transmittance of 83% at 500 nm. However, the problem thereof is that its glass transition temperature is 244° C., which is not high enough and does not fall within the target range.    [Non-Patent Document 1] Macromolecules, Vol. 29, 1996, pp. 7897-7909    [Non-Patent Document 2] Macromolecules, Vol. 24, 1991, pp. 5001-5005    [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-161136 A    [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-046054 A    [Patent Document 3] Japanese Patent Application Laid-Open No. Hei 08-104750 A